This invention relates generally to measurement-while-drilling (MWD) techniques used in oil and gas well drilling. More particularly, this invention relates to a method of estimating and identifying the channel filter characteristics of the drilling fluid communications channel based on surface decoding of the MWD signal.
In the drilling of oil and gas wells, efficient operation of the drilling apparatus, particularly as wells are drilled deeper and offshore activity increases, demands that data of interest to the driller be collected downhole and be sensed and transferred to the surface "continuously" i.e., without the lengthy delays which would be incident to stopping drilling and lowering test instruments down the borehole. In recent years, significant advances have been made in measurement-while-drilling (MWD) technology. For examples of MWD systems for use in the measurement of borehole directional parameters, reference may be had to U.S. Pat. Nos. 3,982,431, 4,013,945 and 4,021,774, all of which are assigned to the assignee of the present invention.
The measurement systems of the above-referenced patents utilize mud pulse telemetry to transmit information from the vicinity of the drill bit to the surface drilling platform. Mud pulse telemetry consists of the transmission of information via a flowing column of drilling fluid, i.e, mud, the information commensurate with the sensed downhole parameters being converted into a binary code of pressure pulses in the drilling fluid within the drill pipe or standpipe which are sensed at the surface. These pressure pulses are produced by periodically modulating the flowing mud column at a point downhole by mechanical means, and the resulting periodic pressure pulses appearing at the surface end of the mud column are detected by a pressure transducer conveniently located in the standpipe. The drilling mud is pumped downwardly through the drill pipe (string) and then back to the surface through the annulus between the drill string and the wall of the well for the purpose of cooling the bit, removing cuttings produced by the operation of the drill bit from the vicinity of the bit and containing the geopressure.
It will be appreciated that accurate decoding of the MWD data signal at the surface is essential to the drilling operation. It will also be appreciated that the MWD data signal will be altered between the mud pulse transmitter and the surface due to the transmission characteristics of the drilling mud in the standpipe. In other words, the mud filled standpipe may be analogized to a channel filter having certain channel characteristics as a transmission medium. In order to obtain accurate and successful decoding of the MWD signal pulse and thereby optimize the MWD communication, there is a need for identifying the mud channel's characteristics as a transmission medium.